A cooling fan module comprises a fan driven by an electric motor. Cooling fan modules are used, for example in vehicles to cool the radiator of the engine of a conventional vehicle or the battery of an electric vehicle.
The motor drive circuit of a conventional two speed cooling fan module has a high-speed branch and a low-speed branch which are connected in parallel. When the high-speed branch is turned on, the motor runs at a high speed. When the low-speed branch is turned on and the high-speed branch is turned off, the motor runs at a low speed. Generally, power is directly connected to an input terminal (for example, a positive electrode) of the motor through the high-speed branch, and the low-speed branch outputs power to the motor terminal at a reduced voltage via a voltage dropping resistor. In this case, if the motor is running at a low speed, the voltage dropping resistor generates a lot of heat, which causes a great loss.
As known to those skilled in the art, a Pulse Width Modulation (PWM) circuit has an adjustable duty ratio, and the average value of an output voltage may be changed by adjusting the duty ratio of the PWM circuit. An improved variable speed cooling fan module is provided, in which the PWM circuit with an adjustable duty ratio is used to replace the conventional high-speed branch and low-speed branch.
In a vehicle radiator cooling system, the PWM circuit is connected to an Engine Control Unit (ECU) of the vehicle through a control terminal, and the duty ratio changes in response to instructions from the ECU in a real-time manner, thus the output voltage is adjusted and the motor is provided with various voltages to operate at different speeds in a low speed range. For example, where the duty ratio is 100%, the PWM circuit is equivalent to the conventional high-speed branch, the motor runs at a high speed; or where the duty ratio is smaller than 100%, the motor runs at a lower speed.
The loss of the PWM circuit described above is smaller than that of the convention circuit with the voltage dropping resistor. However, the overall cost is greatly increased because an additional control circuit (for example, a circuit for adjusting the duty ratio) is included in the PWM circuit. Besides, the conventional high-speed branch and low-speed branch connected in parallel are replaced by a power supply branch in the PWM circuit and the PWM circuit has an additional control terminal, which means that the interface of the cooling fan module adopting the PWM circuit is incompatible with the conventional interface. In the vehicle industry, different components are provided by different providers, hence, the incompatibility of the interface has a serious impact on sales of the cooling fan module adopting the PWM circuit and applications of the cooling fan module.
Therefore, an improved technical solution is desired.